1. Field of the Invention
The present invention relates to a display device, and particularly to an IPS-type liquid crystal display device with excellent viewing angle characteristics.
2. Description of the Related Art
In a liquid crystal display panel used for a liquid crystal display device, there are arranged a TFT substrate on which pixels having pixel electrodes and thin-film transistors (TFTs) are formed in a matrix shape, and an opposed substrate which faces the TFT substrate and on which color filters are formed at positions corresponding to the pixel electrodes of the TFT substrate. In addition, liquid crystal is sandwiched between the TFT substrate and the opposed substrate. The transmittance of light by liquid crystal molecules is controlled for each pixel to form an image.
Liquid crystal display devices are flat and light, and thus have been widely used in various fields. Small-sized liquid crystal display devices have been widely used in cellular phones or DSCs (Digital Still Cameras). There is a problem of viewing angle characteristics in the liquid crystal display device. The viewing angle characteristics are phenomena in which brightness and chromaticity are changed when a screen is viewed from the front direction or oblique direction. An IPS (In plane Switching)-type liquid crystal display device in which liquid crystal molecules are operated by an electric field in the horizontal direction is excellent in the viewing angle characteristics.
There are many kinds of IPS-type liquid crystal display devices. For example, transmission can be increased by an IPS-type liquid crystal display device in which a common electrode or a pixel electrode is formed in a flat shape, a comb-like pixel electrode or common electrode is arranged thereon while sandwiching an insulating film, and liquid crystal molecules are rotated by electric field generated between the pixel electrode and the common electrode. Thus, the IPS-type liquid crystal display device is currently the mainstream.
In such an IPS type, a TFT is formed first and is covered with a passivation film. On the passivation film, formed are the common electrode, the insulating film, the pixel electrode, and the like in a conventional technique. However, the numbers of conductive films and insulating films in the TFT substrate are reduced due to demand of reducing the manufacturing costs.
As another example of the IPS type, FIG. 13 of Japanese Patent Application Laid-Open No. 2009-168878 illustrates a configuration in which a common electrode is formed in the same layer as a gate electrode, and a gate to insulating film and a comb-like pixel electrode are formed while sandwiching a protective insulating film.
FIG. 11 is a plan view for showing an IPS pixel structure targeted by the present invention. In FIG. 11, a pixel is formed in an area surrounded by scanning lines 10 and video signal lines 20. A TFT is formed on the scanning line 10. Specifically, a semiconductor film 103 is formed on the scanning line 10 through a gate insulating film 102, and a drain electrode 104 and a source electrode 105 are formed thereon. In addition, the scanning line 10 also serves as a gate electrode. In the pixel structure as shown in FIG. 12, a pixel electrode 101 connected to the source electrode 105 of the TFT is formed in the lowermost layer, a common electrode 107 is formed in the uppermost layer, and liquid crystal molecules 200 are driven by voltage between the pixel electrode 101 and the common electrode 107.
FIG. 12 is a cross-sectional view taken along the line G-G of FIG. 10. In FIG. 12, the gate electrode 10 as the scanning line 10 and the pixel electrode 101 are formed on the TFT substrate 100 made of glass. The gate electrode 10 is formed using a laminated film of Al and AlMo alloy, and the pixel electrode 101 is formed using ITO (Indium Tin Oxide). The gate insulating film 102 is formed while covering the gate electrode 10 and the pixel electrode 101.
On the gate electrode 10 and the gate insulating film 102, there is formed the semiconductor film 103 made of a-Si on which the drain electrode 104 and the source electrode 105 are formed. The source electrode 105 is connected to the pixel electrode 101 through a through-hole 110 formed in the gate insulating film 102. The through-hole 110 is formed in a horizontally-long shape to reduce contact resistance. An inorganic passivation film 106 is formed while covering the drain electrode 104, the source electrode 105, and the like. On the inorganic passivation film 106, the common electrode 107 is formed. Slits 1071 are formed at the common electrode 107. If voltage is applied between the pixel electrode 101 and the common electrode 107, lines of electric force are generated through the slits 1071 and liquid crystal molecules 200 are rotated by the lines of electric force. Thus, the amount of light penetrating through the liquid crystal layer can be controlled. As described above, the IPS type to which the present invention is applied is largely different from the configuration of the liquid crystal display device described in Japanese Patent Application Laid-Open No. 2009-168878.
In the configuration shown in FIG. 12, the number of layers to be formed is small, and the number of photolithography steps is also small. Thus, the structure shown in FIG. 12 is excellent in manufacturing costs. On the other hand, in the case where pixel capacity is formed between the pixel electrode 101 and the common electrode 107 to suppress a voltage shift caused by changes in gate voltage in the TFT, it is disadvantageously difficult to increase the pixel capacity.
Specifically, the pixel capacity is formed between the pixel electrode 101 and the common electrode 107 in FIG. 12, and the gate insulating film 102 and the inorganic passivation film 106 are provided between the pixel electrode 101 and the common electrode 107. The thicknesses of the gate insulating film 102 and the inorganic passivation film 106 are about 300 nm and 500 nm, respectively, and both films are made of SiN. As described above, the pixel capacity is formed through the insulating film with a thickness of 800 nm in total, and thus cannot be sufficiently increased. Accordingly, pixel voltage caused by ON/OFF of gate voltage is largely affected.
An object of the present invention is to solve the above-described problems and to realize an inexpensive IPS-type liquid crystal display device in which the number of laminated films is small and pixel voltage is less shifted.